1. Field of the Invention
The present invention relates to an electronically controlled automatic transmission capable of sensing a break in an output shaft revolution sensor without the provision of a sensor for sensing a vehicle speed when the break is caused to the output shaft revolution sensor.
2. Description of the Related Art
FIG. 1 is a block diagram conceptually showing the arrangement of a conventional electronically controlled automatic transmission.
In FIG. 1, a control unit 1 for controlling the electronically controlled automatic transmission includes an engine revolution sensor 2, an input shaft revolution sensor 3, an output shaft revolution sensor 4, a vehicle speed reed switch 5 and a selector lever position sensor 6 each connected thereto.
Interfaces (hereinafter, referred to as I/Fs) 1a to 1e are disposed in the control unit 1 to input sensing signals transmitted from the respective sensors (2 to 6) to a CPU 7.
The I/F 1a is an input circuit for inputting the number of revolutions of an engine transmitted from the engine revolution sensor 2 to the CPU 7, and the I/F 1b is an input circuit for inputting the number of revolutions of the input shaft of the transmission transmitted from the input shaft revolution sensor 3 to the CPU 7.
The I/F 1c is an input circuit for inputting the number of revolutions of the output shaft of the transmission transmitted from the output shaft revolution sensor 4 to the CPU 7, and the I/F 1d is an input circuit for inputting the position of a selector lever transmitted from the selector lever position sensor 6 to the CPU 7.
In the conventional electronically controlled automatic transmission as described above, a gear ratio is selected based on the number of revolutions of the output shaft of the transmission and an amount of depression of an accelerator pedal, thereafter a vehicle speed is changed by driving a speed change solenoid (not shown).
FIG. 2 shows a general gear ratio map of a stage-type electronically controlled automatic transmission.
FIG. 3 shows a general gear ratio map of a non-stage type electronically controlled automatic transmission.
FIG. 4 shows the relationship between the number of revolutions of the input shaft of a transmission and the number of revolutions of the output shaft thereof in a shift-down operation.
Generally, when the number of revolutions of the output shaft of an automatic transmission is low, a gear ratio on a lowest speed side is selected in view of acceleration ability when an accelerator pedal is depressed.
In the electronically controlled automatic transmission arranged as described above, for example, when a failure such as a brake and the like is caused to the output shaft revolution sensor 4, there arises a situation that a gear ratio on a lowest speed side is selected while the vehicle travels at a high speed, since it is erroneously determined that a vehicle is stoped regardless of the fact that it is actually traveling.
In such a case, since it is abruptly determined that the number of revolutions of the output shaft is zero as shown in FIG. 4, the difference between the number of revolutions of the input shaft and the number of revolutions of the engine is increased. Therefore, there is a possibility of occurrence of an excessive engine braking and an overrun of the engine.
Since the excessive engine braking and the overrun of the engine make the traveling state of the vehicle unstable and may break the vehicle, the failure of the output shaft revolution sensor 4 must be monitored from the viewpoint of securing safety in traveling vehicles and excluding sources of the failure of the vehicles.
For example, according to the invention disclosed in Japanese Unexamined Patent Publication No. 60-11753, a vehicle speed reed switch 5 is provided as a vehicle speed sensor independently of an output shaft revolution sensor 4 to monitor the failure of the output shaft revolution sensor 4 such as a break and the like. The invention has an advantage that the safety can be enhanced by the provision of the vehicle speed reed switch 5 for monitoring the failure of the output shaft revolution sensor 4, at the time the output shaft revolution sensor 4 fails.
There is also available an electronically controlled automatic transmission for monitoring an abnormal state of an output shaft revolution sensor by simply comparing the number of revolutions of the input shaft of a transmission with the number of revolutions of the output shaft thereof as disclosed in Japanese Examined Patent Publication No. 7-107424.
However, since the vehicle speed reed switches 5 used in the electronically controlled automatic transmission disclosed in Japanese Unexamined Patent Publication No. 60-11753 are different for respective types of vehicles, they cannot be mass-produced and are expensive.
Accordingly, there is a problem that it is contrary to a request for cost reduction to provide the expensive vehicle speed reed switch 5 only for monitoring failure.
In the invention disclosed in Japanese Examined Patent Publication No. 7-107424, an abnormal state caused in any of the input shaft revolution sensor and the output shaft revolution sensor can be sensed by the comparison of the number of revolutions of the input shaft with the number of revolutions of the output shaft. However, the invention has a problem that it cannot specify the sensor in which the abnormal state is caused and it cannot determine that the abnormal state is not caused in the sensors and caused by the failure in the transmission (slip of a friction member, a faulty operation of a shift solenoid, a faulty operation of a shift valve, and the like), and thus a failure cannot be analyzed.
Further, when the same connectors are used for the connection of the input shaft revolution sensor and the output shaft revolution sensor, there is also a problem that when the connectors are pulled out, a failure itself cannot be sensed.